Kindlins have emerged as a family of integrin binding proteins with a key role in integrin activation. A recent study from our lab identified a causative mutation in Kindlin 3 (K3) in sibling patients with leukocyte adhesion deficiency II (LADIII; Malinin et. al Nature Medicine 2009). The patients exhibited severe haemostatic and immune dysfunctions that were rescued by bone marrow transplantation. The mutation introduced a premature stop codon in K3 leading to loss of platelet and leukocyte integrins capacity to get activated. Little is known about the mechanisms of K3 mediated signaling. Mechanistically, kindlins bind directly to integrin cytoplasmic tails through a conserved FERM domain at their C-terminal half. It is not clear however whether kindlins play a regulatory or a permissive role in integrin activation. Our preliminary observations demonstrate that K3 preferentially localizes to the edges of membrane protrusions supporting a regulatory function. Interestingly, K3 subcellular localization differs from that of K2, suggesting non-overlapping functions of the two proteins. Furthermore, our preliminary results show requirement of K3 N-terminal portion for membrane association and integrin activation. The main hypothesis of this proposal is that in response to integrin activating signals K3 translocate to the plasma membrane to allow integrin activation; and that K2 and K3 have differential dynamics of membrane association underlying their differential properties in integrin activation. To test this hypothesis we will 1) map residues required for K3 membrane association and establish the role of K3 membrane binding in integrin activation using structure based mutagenesis; 2) monitor the dynamics of K2 and K3 subcellular localization during cell migration and evaluate the role of the membrane binding domains in differential localization; 3) test whether physiological integrin activating stimuli target K3 to the membrane and evaluate the importance of targeting using physiologically relevant systems. These studies will significantly expand understanding of the mechanisms of integrin activation. Insights from this study will promote the development of therapeutic strategies to inhibit integrin activation for treatment of a number of pathological conditions including coagulative disorders, stroke, ischemic heart disease, arteriosclerosis, immune disorders, cancer and metastasis.